As conventional fixing methods and devices which have heretofore been employed in the image forming apparatus of the kind, for example, those disclosed in Japanese Patent Publication No. 59-4699, Japanese Patent application Laid-Open Nos. 59-74579 and 60-129768 are known. One example of the known fixing method and device is illustrated in FIG. 9, where the essential part is composed of a heat roll (100) which has a heat source in the inside thereof and which rotates to the direction of the indicated arrow, a pressure roll (101) which is arranged to be kept in direct contact with the heat roll (100) under pressure and which rotates to the direction of the indicated arrow, and a toner release supply means (102), which is fitted to the heat roll (100) to supply a toner release for prevention of toner offset to the outer surface of the heat roll (100). In accordance with the illustrated method and device, a copying paper (103) carrying a non-fixed toner image thereon is inserted and passed between the heat roll (100) and the pressure roll (101) to thereby fix the toner image on the paper.
Precisely, the heat roll (100) is composed of a base roll (104) having a heat source in the inside thereof, an inner elastic material layer (105) as provided on the base roll (104), and an outer elastic material layer (106) which is made of an elastic material, such as a fluorine-containing rubber, having an affinity with the toner release for prevention of toner offset and having an abrasion-resistant property, and which is provided on the inner elastic material layer (105). The heat roll (100) having such a constitution is brought into contact with the copying paper (103) under a pertinent pressure and with a pertinent contact width because of the elastic action of the inner elastic material layer (105), while the toner offset phenomenon is prevented by the function of the toner release to be supplied to the outer elastic material layer (106).
As the toner release to be coated on the surface of the heat roll (100), for example, a polydimethylsiloxane having an ordinary silicone oil base is generally employed.
Recently, Viton Rubber (trade name by DuPont), which is a fluorine-containing rubber having excellent heat-resistance and abrasion-resistance, is often used as the outer elastic material layer (106) of the heat roll (100). However, since Viton Rubber has a poor affinity with the polydimethylsiloxane as a toner release, there is a problem that the outer elastic material layer made of the rubber could not have a sufficient releasability.
In order to overcome the problem, use of a mercapto-modified silicone oil, which has a mercapto group having a high affinity with Viton Rubber to be used as the outer elastic material layer (106) of the heat roll (100), as a toner release has been proposed, and the oil has actually been put to practical use as the agent.
However, the above-mentioned prior art has the following problems.
Precisely, where such a mercapto-modified silicone oil having a high affinity with Viton Rubber to be used as the outer elastic material layer of a heat roll is used as a tone release, the mercapto-modified silicone oil would gel by chemical reaction under heat during use for a long period of time to thereby lower the heat-resistance of the heat roll.
Additionally, where the surface temperature of the heat roll is set high so as to improve the fixability of the toner image to be formed on a copying medium, the mercapto-modified silicone oil would be deteriorated to often shorten the life of the heat roll.
In a color duplicator, as formation of images is effected by the user of plural toners such as cyan, magenta and yellow toners, the amount of the toners to be fixed is large. Further, since toners having a low melting point are used in the case for the purpose of improving the color forming property, such toners are generally highly viscous so that the toner release to be used in the case is needed to have a high releasability. Despite of the situation, the conventional toner releases applicable to the case have a problem that the releasability is insufficient.